Solution Structure of Human Pex5.Pex14.PTS1 Protein Complexes Obtained by Small Angle X-ray Scattering

The Pex5p receptor recognizes newly synthesized peroxisomal matrix proteins which have a C-terminal peroxisomal targeting signal to the peroxisome. After docking to protein complexes on the membrane, these proteins are translocated across the membrane. The docking mechanism remains unclear, as no structural data on the multicomponent docking complex are available. As the interaction of the cargo-loaded Pex5p receptor and the peroxisomal membrane protein Pex14p is the essential primary docking step, we have investigated the solution structure of these complexes by small angle x-ray scattering and static light scattering. Titration studies yielded a 1:6 stoichiometry for the Pex5p.Pex14p complex, and low resolution structural models were reconstructed from the x-ray scattering data. The free full-length human Pex5p is monomeric in solution, with an elongated, partially unfolded N-terminal domain. The model of the complex reveals that the N terminus of Pex5p remains extended in the presence of cargo and Pex14p, the latter proteins being significantly intermingled with the Pex5p moiety. These results suggest that the extended structure of Pex5p may play a role in interactions with other substrates such as lipids and membrane proteins during the formation of functional multiprotein complexes.